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Number theory Difficulty 6.0 AIME, harder Prove it Italy

Determine all positive integers mm for which both 25m+103m+1\frac{2 \cdot 5^{m}+10}{3^{m}+1} and 9m+15m+5\frac{9^{m}+1}{5^{m}+5} are integers.

Solution

Notice that if a=25m+103m+1a=\frac{2 \cdot 5^{m}+10}{3^{m}+1} and b=9m+15m+5b=\frac{9^{m}+1}{5^{m}+5} are both integers, then their product,
ab=(25m+10)(9m+1)(3m+1)(5m+5) a b=\frac{\left(2 \cdot 5^{m}+10\right)\left(9^{m}+1\right)}{\left(3^{m}+1\right)\left(5^{m}+5\right)}
is also an integer.
Collecting now a factor of two in the numerator, this expression can be rewritten as
2(5m+5)(9m+1)(3m+1)(5m+5)=2(9m+1)3m+1. \frac{2\left(5^{m}+5\right)\left(9^{m}+1\right)}{\left(3^{m}+1\right)\left(5^{m}+5\right)}=\frac{2\left(9^{m}+1\right)}{3^{m}+1} .
Let us now see for which mm this expression is an integer. Noting that 9m+1=(3m+1)223m9^{m}+1=\left(3^{m}+1\right)^{2}-2 \cdot 3^{m}, we can further transform the product aba b into the form
ab=2(9m+1)3m+1=2(3m+1)223m3m+1=2(3m+1)43m3m+1 a b=2 \cdot \frac{\left(9^{m}+1\right)}{3^{m}+1}=2 \cdot \frac{\left(3^{m}+1\right)^{2}-2 \cdot 3^{m}}{3^{m}+1}=2\left(3^{m}+1\right)-4 \frac{3^{m}}{3^{m}+1}
which is an integer if and only if 43m3m+14 \frac{3^{m}}{3^{m}+1} is an integer.
We now prove that 2<43m3m+1<42<4 \frac{3^{m}}{3^{m}+1}<4.
Indeed, since m1m \geq 1, we have 3m33^{m} \geq 3, and therefore 23m+2<23m+23m<4(3m+1)2 \cdot 3^{m}+2<2 \cdot 3^{m}+2 \cdot 3^{m}<4 \cdot\left(3^{m}+1\right); dividing these three terms by 3m+13^{m}+1 we then find 2<43m3m+1<42<4 \frac{3^{m}}{3^{m}+1}<4 as desired.
Therefore, if 43m3m+14 \frac{3^{m}}{3^{m}+1} is an integer, it must necessarily be equal to 3, since it is strictly between 2 and 4.
Imposing this equality we obtain 43m=3(3m+1)3m=3m=14 \cdot 3^{m}=3 \cdot\left(3^{m}+1\right) \Rightarrow 3^{m}=3 \Rightarrow m=1.
Therefore, if there exists an mm with the required property, it must be equal to 1. On the other hand, it is easy to verify that 251+1031+1=5\frac{2 \cdot 5^{1}+10}{3^{1}+1}=5 and 91+151+5=1\frac{9^{1}+1}{5^{1}+5}=1 are both integers. We can therefore conclude that m=1m=1 is indeed a solution and is the only one.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.